The present invention relates to novel methods for monitoring and guiding therapeutic suppression of parathyroid hormone in renal patients having secondary hyperparathyroidism. One determines and monitors the level of cyclase activating parathyroid hormone and cyclase inactive parathyroid hormone in the renal patient. The parathyroid hormone suppressing therapeutic is administered to the patient so as to minimize the level of cyclase inactive parathyroid hormone.
Calcium plays an indispensable role in cell permeability, the formation of bones and teeth, blood coagulation, transmission of nerve impulse, and normal muscle contraction. The concentration of calcium ions in the blood is, along with calcitrol and calcitonin, regulated mainly by parathyroid hormone (PTH). Although calcium intake and excretion may vary, PTH serves through a feedback mechanism to maintain a steady concentration of calcium in cells and surrounding fluids. When serum calcium lowers, the parathyroid glands secrete PTH, affecting the release of stored calcium. When serum calcium increases, stored calcium release is retarded through lowered secretions of PTH.
The complete form of human PTH, sometimes referred to in the art as hPTH but referred to in the present invention as cyclase activating PTH or whole PTH, is a unique 84 amino acid peptide (SEQ ID NO.1), as is shown in FIG. 1. Researchers have found that this peptide has an anabolic effect on bone that involves a domain for protein kinase C activation (amino acid residues 28 to 34) as well as a domain for adenylate cyclase activation (amino acid residues 1 to 7). However, various catabolic forms of clipped or fragmented PTH peptides also are found in circulation, most likely formed by intraglandular or peripheral metabolism. For example, whole PTH can be cleaved between amino acids 34 and 35 to produce a (1-34) PTH N-terminal fragment and a (35-84) PTH C-terminal fragment. Likewise, clipping can occur between either amino acids 36 and 37 or 37 and 38. Recently, a large PTH fragment referred to as xe2x80x9cnon-(1-84) PTHxe2x80x9d has been disclosed which is clipped closer to the N-terminal end of PTH. (See R. LePage et alia, xe2x80x9cA non-(1-84) circulating parathyroid hormone (PTH) fragment interferes significantly with intact PTH commercial assay measurements in uremic samplesxe2x80x9d Clin Chem (1998); 44: 805-810.)
The clinical need for accurate measurement of PTH is well demonstrated. Serum PTH level is one of the most important indices for patients with the following diseases: familial hypocalciuria; hypercalcemia; multiple endocrine neoplasia types I and II; osteoporosis; Paget""s bone disease; primary hyperparathyroidismxe2x80x94caused by primary hyperplasia or adenoma of the parathyroid glands; pseudohypoparathyroidism; and renal failure, which can cause secondary hyperparathyroidism.
PTH plays a role in the course of disease in a patient with chronic renal failure. Renal osteodystrophy (RO) is a complex skeletal disease comprising osteitis fibrosa cystica (caused by PTH excess), osteomalaciaxe2x80x94unmineralized bone matrix (caused by vitamin D deficiency), extraskeletal calcification/ossification (caused by abnormal calcium and phosphorus metabolism), and adynamic low bone turnover disease (contributed to by PTH suppression). Chronic renal failure patients can develop RO. Failing kidneys increase serum phosphorus hyperphosphoremia) and decrease 1,25-dihydroxyvitamin D (1,25-D) production by the kidney. The former results in secondary hyperparathyroidism from decreased gastrointestinal calcium absorption and osteitis fibrosa cystica from increased PTH in response to an increase in serum phosphorus. The later causes hypocalcemia and osteomalacia. With the onset of secondary hyperparathyroidism, the parathyroid gland becomes less responsive to its hormonal regulators because of decreased expression of its calcium and vitamin D receptors. Serum calcium drops. RO can lead to digital gangrene, bone pain, bone fractures, and muscle weakness.
For chronic renal failure patients with secondary hyperparathyroidism, a number of different therapeutic treatments are available. One can administer calcium carbonate so as to directly adjust the available calcium ion level. However, with the increasing incidence of ectopoic calcification, increasing calcium intake is often not desirable. One can administer calcimimetics, such as AMG073 made by Amgen, Inc. of Thousand Oaks, Calif. However, AMG073 has been shown to have some hypercalcemic effect and has not been approved for use in the USA. One can administer vitamin D analogues, (such as the Calcijex or Zemplar brands made by Abbott Labs of Abbott Park, Ill. or the Rocaltrol brand made by Roche Laboratories of Basle, Switzerland), so as to lower PTH. However, researchers have found that vitamin D analogues can oversuppress PTH, thereby leading to adynamic low bone turnover disease setting the patient at risk of ectopic and vascular calcification. (See the package insert for Zemplar, Abbott Reference 06-9998-R1-Rev, April 1998. See the package insert for Rocaltrol, Roche Laboratories, inc. November 1998 Product identification Guide, page 334.)
Researchers have also found that a large circulating PTH fragment (cyclase inactive parathyroid hormone) functions as a naturally occurring PTH antagonist. Cyclase inactive PTH has been found to be useful, alongside whole PTH, as an indicator in separating untreated end stage renal disease (ESRD) patients with high bone turnover from those with adynamic low bone turnover. (See Faugere, M. C. et alia. xe2x80x9cImproved Assessment of Bone Turnover by the PTH 1-84/largeC-PTH fragments ratio in ESRD patientsxe2x80x9d, Kidney International 2001; 60: 1460-1468.) Moreover, researchers have found that cyclase inactive PTH can cause adynamic low bone turnover by inhibiting the formation of osteoclasts, bone resorption, and bone turnover. (See Divieti P. et alia, xe2x80x9cIn vitro Inhibition of Bone Resorption by Human PTH (7-84)xe2x80x9d J. Bone Miner Res 2001:Suppl 1, S307. See also Faugere, M. C. et alia, xe2x80x9cThe Effects of PTH (1-84) on bone turnover are Antagonized by PTH (7-84) in Thyroparathyroidectomized and Nephrectomized Ratsxe2x80x9d; J Am Soc Nephrol 12:2001, 764A.)
Determining circulating biologically active PTH levels in humans has been challenging. One major problem is that PTH is found at low levels, normally 10 pg/mL to 65 pg/mL. Coupled with extremely low circulating levels is the problem of the heterogeneity of PTH and its many circulating fragments. In many cases, immunoassays have faced substantial and significant interference from circulating PTH fragments. For example, some commercially available PTH kits have almost 100% cross-reactivity with the non-(1-84) PTH fragment, (see the LePage article).
PTH immunoassays have varied over the years. One early approach is a double antibody precipitation immunoassay found in U.S. Pat. No. 4,369,138 to Arnold W. Lindall et alia. A first antibody has a high affinity for a (65-84) PTH fragment. A radioactive labeled (65-84) PTH peptide is added to the sample with the first antibody to compete for the endogenous unlabeled peptide. A second antibody is added which binds to any first antibody and radioactive labeled PTH fragment complex, thereby forming a precipitate. Both precipitate and supernatant can be measured for radioactive activity, and endogenous PTH levels can be calculated therefrom.
In an effort to overcome PTH fragment interference, immunoradiometric two-site assays for intact PTH (I-PTH) have been introduced, such as Allegro(copyright) Intact PTH assay by the Nichol""s Institute of San Juan Capistrano, Calif. In one version, a capture antibody specifically binds to the C-terminal portion of hPTH while a labeled antibody specifically binds to the N-terminal portion of the captured hPTH. In another, two monoclonal antibodies were used, both of which attached to the N-terminal portion of hPTH. Unfortunately, these assays have problems in that they measure but do not discriminate between whole PTH and non-whole PTH peptide fragments. This inability comes to the fore in hyperparathyroid patients and renal failure patients who have significant endogenous concentrations of large, non-whole PTH fragments.
Recently, Scantibodies Laboratory, Inc. of Santee, Calif. USA introduced a series of novel kits that allow for the accurate measurement of both cyclase activating PTH and cyclase inactive PTH, (a large, non-whole PTH peptide fragment having an amino acid sequence from between (SEQ ID No.2 [PTH2-84]) and (SEQ ID No.3 [PTH34-84]). The cyclase activating PTH assay is a direct measurement, while the cyclase inactive PTH assay is a calculated value from the difference of the cyclase activating PTH assay value and a total PTH (both cyclase activating PTH and cyclase inactive PTH) assay value. A number of unexpected advantages have become available to the physician, including the first non-invasive method for assisting in the differentiation of secondary hyperparathyroid patients with HBT and ALBT.
The present invention relates to novel methods for monitoring and guiding therapeutic suppression of parathyroid hormone in renal patients having secondary hyperparathyroidism. One determines and monitors the level of cyclase activating parathyroid hormone and cyclase inactive parathyroid hormone in the renal patient. The parathyroid hormone suppressing therapeutic is administered to the patient so as to minimize the level of cyclase inactive parathyroid hormone.
Secondary hyperparathyroidism is a common disease in renal compromised patients, especially those with ESRD. Virtually all ESRD patients have bone disease and mineral metabolism disorders, either high bone turnover disease or adynamic low bone turnover disease. Elevated levels of whole PTH (with respect to cyclase inactive PTH) lead to high bone turnover disease (HBT). Elevated levels of cyclase inactive PTH (with respect to whole PTH) lead to adynamic low bone turnover disease (ALBT). The more serious of the two is ALBT. Ectopic tissue calcification results in vascular stenosis (including occlusion of coronary arteries) and aortic rigidity. ALBT patients are more likely to die due to a circulatory system failure, such as myocardial infarction than those with HBT. One reason is the difficulty in finding a reliable therapeutic indicator for ALBT or for therapeutic oversuppression that leads to ALBT. Another reason is that due to the lack of the reliable indicator, whole PTH suppressant therapy can inadvertently lead to ALBT due to PTH oversuppression. Difficulty with implementing this therapy can be seen in the setting of at least ten different PTH target recommendations within the past decade. The net result of this uncertainty in therapeutic indicators is that the incidence of vascular calcification has been reported to be 88% for ESRD patients, leading to a slow and agonizing death. (See Goodman, W. et alia, xe2x80x9cCoronary Artery Calcification in Young Adults with End Stage Renal Disease Who Are Undergoing Dialysisxe2x80x9d; NEJM 2000, May 18; 342:20, 1478-1483.)
A novel finding of the present invention is that while whole PTH only decreases in response to the administration of whole PTH suppressants, cyclase inactive PTH does not. For an untreated renal patient with an elevated whole PTH, the administration of a PTH suppressant will also suppress cyclase inactive PTH initially. However, as the PTH suppressant dosage increases, the level of cyclase inactive PTH will reach a minimal level and then start to increase before the level of whole PTH stops decreasing. So as to avoid inducing ALBT in such patients, the administration of PTH suppressant should be adjusted so as to stay about that minimal cyclase inactive PTH level. Ordinarily skilled artisans know that this level may vary from patient to patient, but can determine what is best for a particular patient through monitoring the response to therapy.
Preferably, whole PTH is measured directly using an assay that does not detect cyclase inactive PTH either in blood, plasma, or serum. Scantibodies Laboratory, Inc. of Santee, Calif. makes such an assay.
Cyclase inactive PTH should be measured using an assay that either directly detects cyclase inactive PTH (but not whole PTH) or indirectly through a total PTH measurement. An indirect measurement subtracts the whole PTH value from the total PTH value, deriving the cyclase inactive PTH value. Thus, one should use a total PTH assay that is designed to have essentially 100% cross-reactivivity with cyclase inactive PTH. Scantibodies Laboratory, Inc. of Santee, Calif. makes such an assay.
A novel finding of the present invention is that while whole PTH only decreases in response to the administration of whole PTH suppressants, cyclase inactive PTH does not. For an untreated renal patient with an elevated whole PTH, the administration of a PTH suppressant will also suppress cyclase inactive PTH initially. However, as the PTH suppressant dosage increases, the level of cyclase inactive PTH will reach a minimal level and then start to increase before the level of whole PTH stops decreasing. So as to avoid inducing ALBT in such patients, the administration of PTH suppressant should be adjusted so as to stay about that minimal cyclase inactive PTH level. Ordinarily skilled artisans know that this level may vary from patient to patient, but can determine what is best for a particular patient through monitoring the response to therapy.